This invention is generally in the field of interferometry, and relates to a system and method for interferometric measurements used for inspecting samples. The invention can be particularly used with a microscope or other imaging systems to acquire quantitative inspection of transparent, semi-transparent or reflective samples.
Interferometric microscopy, also known as wide-field interferometric phase microscopy (IPM) or digital holographic microscopy can be used to simultaneously record the quantitative spatial profiles of both the amplitude and the phase of the measured samples. Using interferometric microscopy, time recording of the phase profile can yield remarkable optical thickness or optical-path-delay stability of less than a nanometer, with acquisition rates of several thousands of full frames per second, and without the need for using contrast agents such as florescence dyes. As the technique provides the optical thickness per each spatial point on the sample, various relevant morphological and mechanical parameters of the sample can be obtained in a non-contact, label-free manner. IPM can be utilized for a wide range of applications including biological cells investigations, surface measurements, biometry, and others. IPM uses interference to record the complex wave front (amplitude and phase) of the light interacted with the sample. For biological and medical applications, the ability to record the sample quantitative phase enables the user to see cells and organisms, which are otherwise transparent due to the cell low absorption and scattering of the transmitted light.
These unique advantages could be attractive for many clinical applications, so many IPM setups were presented over the years, and they can be divided into various groups, such as setups that use common-path geometry [7-9] or separated reference and sample beam geometry [10], setups that use high-coherence source [6] or low-coherence source [11-13], setups that use on-axis (inline) geometry or off-axis geometry [4, 5, 10-14]. However, not many options are available for commercial interferometric microscopes compared to other microscopy techniques, and this tool is mostly used by optical and biomedical engineers for research purposes. One reason for this is the difficulty to obtain high-quality and stable interference pattern with modest and portable equipment and without the need for an expert user. The commonly-used interferometric setups are usually constructed in open and custom-built microscopes and operated by a user with knowledge in optics. To ensure the stability of the interference pattern, the entire system is positioned on an optical table to avoid mechanical vibrations and is boxed inside an enclosure to avoid differential air perturbations between the interferometric arms.
Techniques aimed at or enabling higher stability of the interference pattern with compact and portable designs have been developed. One of these systems is the interferometric chamber (InCh) microscope [1]. In this system, all the interferometric elements are encapsulated into a single, rigid and factory-designed reflective chamber. Although this system uses common-path geometry (and thus can operate without an optical table), it can still create off-axis interferograms of the sample (and thus only one frame is required for acquiring the amplitude and the phase profiles of the sample, which is suitable for highly dynamic samples). However, the InCh microscope cannot use high magnifications due to the fact that the microscope objective needs to collect the tilted reference beam. In addition, this microscope requires highly-coherent illumination sources since the optical path difference between the reference and the sample beams are twice the optical thickness of the chamber. A similar technique is described in US 2011/0242543. The system includes a light source for generating an illumination beam that propagates towards a sample. A sample holder may hold the sample and include a partially reflective cover for allowing a first portion of the illumination beam to pass therethrough to interact with the sample to produce a sample beam that propagates substantially along an optical axis. The cover may be oriented at an angle for reflecting a second portion of the illumination beam to produce a reference beam that propagates at a predetermined angle with respect to the optical axis. An imaging module may redirect the reference beam towards the optical axis at a detection plane. A detector may intercept the sample and reference beams and may generate a holographic representation of the sample based on the beams.
Other setups for common-path or self-interference interferometry have been presented [2-5]. In one type of setups, a diffraction grating or other specialized optical elements are used, whereas in another type of setups, a Michelson interferometer in the output of a microscope is used, so that the sample beam interferes with itself, with the limitation that half of the sample has to be empty. Many of the known interferometric setups, however, have the same main drawbacks of bulkiness, non-portability and the requirement for specific optical skills to align and use them. These shortcomings cause this technology to largely remain in optical research laboratories, and thus it is not very common in the industry or in clinics.